This invention relates to high pressure discharge lamps and, more particularly, to light sources wherein the restart time after a momentary power interruption is reduced.
High pressure discharge lamps, such as high pressure sodium, high pressure mercury vapor, and metal halide lamps, provide significantly higher efficiencies than incandescent lamps and are widely used for general lighting purposes. An inherent disadvantage of high pressure discharge lamps is the warm-up period of several minutes during which only a low level of illumination is available. By comparison, incandescent and fluorescent lamps provide full light output in a few seconds or less. The warm-up period or cold-start delay associated with high pressure discharge lamps is due to the necessity for the fill material to be vaporized and the discharge tube to be warmed up before full light output is attained. Furthermore, when power to the lamp is momentarily interrupted, the discharge is extinguished and cannot be re-initiated until the lamp cools off and the pressure in the lamp is reduced. After the discharge is re-ignited, the warm-up period described above must be repeated before the lamp again reaches full light output. The hot restart delay is thus longer than the cold-start delay.
The hot restart delay associated with high pressure discharge lamps is unacceptable in many applications. When high pressure discharge lamps are used in conjunction with heavy electrical equipment, for example, in mines, the equipment can generate power line transients which extinguish the discharge lamps and illumination is lost for several minutes. Temporary power outages and transients from other sources can also cause a loss of illumination from high pressure discharge lamps for several minutes.
It is known to use standby incandescent filaments to provide illumination during the hot restart delay period associated with high pressure discharge lamps. However, additional circuitry is required to energize the incandescent filaments at the proper time. Hot discharge lamps can be restarted by applying a high voltage for a short time. However, additional circuitry is required to apply high voltage to the discharge lamp at the proper time.
While the hot restart delay of high pressure discharge lamps has been discussed in connection with electroded discharge lamps, hot restart delays also occur in high pressure electrodeless lamps powered by high frequency power. Electrodeless lamps can be greatly improved by a reduction or elimination of the hot restart delay associated therewith.